I know, I have that problem, too. I am a 2b 2c, and my hair gets flat and straighter in the winter because there is much less humidity in the air. My solution is to moisturize a lot with deep conditioning and oil, and to use to steam to get moisture back into my hair, using Q Redew. I also use extra spray gel by Deva to get more curl and definition. February is the worst month for dry air.
why does my hair loses its volume in winter?
in the winter girls think the cold weather causes damage, but its mostly the big jackets, scarves, hats and other things we wear that are the culprits of damage, and also we wear our hair in buns and stuff to "protect" it. I think you may be over moisturizing and over touching your hair, the best way to cut volume is to go over board with conditioning products. The less product i use the more volume i get, so maybe cut back on the conditioner and see what happens.
Lack of humidity. Add as much moisture as you can and use humectants to draw whatever moisture is in the air to your hair to give it more plump.